With 200 strings, each with between 100 to 300 lbs. of tension most pianos have over 30,000 lbs. of tension on the frame of the instrument. To properly tune a piano each of these strings must first be brought to correct tension, then every string must be carefully matched with its neighbors, usually several times before they will hold a steady pitch. Piano strings slowly stretch, throughout their life. Humidity changes swells and shrinks wood, changing tension and pitch, even without use a piano will usually not stay in perfect tune longer than six months. Annual tunings are usually considered a minimum service schedule for most pianos. Instruments that are played hard or have large temperature and humidity changes usually require more frequent tunings. Professional instruments are often tuned monthly, studio instruments may even be tuned daily. Badly neglected pianos may require additional tuning (pitch-raising) to be properly tuned.